[gPXE] Port Mirroring Solution

[Jonathan Andrews]

On Mon, 2010-06-28 at 12:41 -0400, Marty Connor wrote:
> On 6/28/10 12:16 PM, Robert Boerner wrote:
> > Since low cost switches with port mirroring were being discussed
> > recently, I thought I would throw my two cents in:
> > 
> > http://www.dual-comm.com/port-mirroring-LAN_switch.htm
> > 
> > This is a USB powered 10/100 switch that mirrors port 1 to port 5.
> > Only $49. They have a gigabit version as well. Although I have not
> > used them myself, someone I used to do work with has:
> > 
> > http://aplawrence.com/Security/dualcomm.html
> 
> Thanks for the link Robert,
> 
> This looks like another great option if 10/100 is sufficient for
> debugging purposes.
> 
> These devices, coupled with WireShark or tcpdump make a great debugging
> tool.
> 
> Say, if anyone is interested in writing a nice appnote
> 
>    ( http://etherboot.org/wiki/appnotes )	
> 
> with screenshots and examples of how to use a port-mirroring switch to
> do a packet capture with WireShark and tcpdump for debugging gPXE, I
> would be happy to donate such a switch as thanks. :)
> 
> What I have in mind is an article we can point people to to help get
> them up to speed with debugging networking problems.
> 
> Please contact me off-list if you are interested.
> 
Nice find, useful info.

I keep an old HUB for this type of job.

I used to use a dual faceplate and link DATA- DATA+ CLK- CLK+ from one
side to the other with 330ohm resistors.  With a short cable length you
can now plug two PCS into one switch port.  This works a treat with
older kit but I have no idea how the autodetection will treat this setup
at 100MB or worse GB.  This setup will unbalance the single port, but
works :-)

Twister pair still honours "collision detect multiple access" CDMA, so
connecting two or more transceivers is an electrical problem only, well
before auto speed negotiation was invented that is .... 

A true passive splitter will work (has own transformers) but they are
very rare. Most "two to one" adaptors just use the second set of
conductors on the twisted pair cable.
http://en.wikipedia.org/wiki/Network_tap

This is a useful approach if all else fails and possibly the only way
for full duplex :
http://hackaday.com/2008/09/14/passive-networking-tap/


Cheers,
Jon